1. Field of the Invention
The present invention relates to a semiconductor device including an oxide semiconductor, a display device including the semiconductor device, and a manufacturing method thereof.
2. Description of the Related Art
Various metal oxides are used for a variety of applications. Indium oxide is a well-known material and is used as a light-transmitting electrode material which is necessary for liquid crystal displays and the like.
Some metal oxides have semiconductor characteristics. Metal oxides having semiconductor characteristics are a kind of compound semiconductor. The compound semiconductor is a semiconductor formed using two or more kinds of atoms bonded together. In general, metal oxides become insulators. However, it is known that metal oxides become semiconductors depending on the combination of elements included in the metal oxides.
For example, it is known that tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like are metal oxides which have semiconductor characteristics. A thin film transistor in which a transparent semiconductor layer which is formed using such a metal oxide serves as a channel formation region is disclosed (Patent Documents 1 to 4 and Non-Patent Document 1).
Further, not only simple (unitary) oxides but also multiple oxides are known as metal oxides. For example, InGaO3(ZnO)m (m is a natural number) which is a homologous compound is a known material (Non-Patent Documents 2 to 4).
Furthermore, it is confirmed that such an In—Ga—Zn-based oxide is applicable to a channel layer of a thin film transistor (also referred to as a “TFT”) (Patent Document 5 and Non-Patent Documents 5 and 6).
In a conventional technique, amorphous silicon or polycrystalline silicon has been used for a thin film transistor (a TFT) provided for each pixel of an active matrix liquid crystal display. However, in place of these silicon materials, attention has been attracted to a technique for manufacturing a thin film transistor including the aforementioned metal oxide semiconductor. Examples of the techniques are disclosed in Patent Document 6 and Patent Document 7, where a thin film transistor is manufactured with zinc oxide or an In—Ga—Zn—O-based oxide semiconductor for a metal oxide semiconductor film and is used as a switching element or the like of an image display device.